How adjust a tractor belt, the tractor model is 917.255940, the belt is to make the blades work, thanks.

Hi,
There is no adjustment per se. One thing that often happens is that rust/corrosion prevents the moveable parts (spring-loaded idlers) from fully tensioning the belt. It is usually a shouldered bushing that is held by a bolt. It can be taken apart, cleaned, lubed and reassembled
Manual not readily available, but parts breakdown found in Denmark at:
http://v-import-manual.s-9.dk/data/archive/files/255940.pdf
Peace,
Clarke